class_docstr: Data structure to store backdoor variables between 2 nodes.
code: def __str__(self):
      string = ""
      string += "Blocked: " + str(self._is_blocked) + "\n"
      if not self._is_blocked:
          condition_vars = [str(s) for s in self._condition_vars]
          string += (
              "To block path, condition on: "
              + ",".join(condition_vars)
              + "\n"
          )
      return string
  
imports: import networkx
  from dowhy.utils.graph_operations import adjacency_matrix_to_adjacency_list
label_desc: Use image node_id 6 for calling the NodePair obj's underlying member method code with example usage: obj.__str__() and returns: string